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Loans Held for Investment - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Percent of private loans indexed to LIBOR | 24.00% |  | 33.00% |  |  |
Tier 1 of government guarantee (at least) | 97.00% |  |  | 97.00% |  |
Tier 2 of government guarantee |  |  |  |  | 98.00% |
Tier 3 of government guarantee |  |  |  |  |  | 100.00%
Estimated weighted average life of student loans | 5 years 4 months 24 days |  | 5 years |  |  |
Private Education Loans |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Gain on sales of loans, net | $ 255 | $ 128 |  |  |  |
Proceeds from sale of loans receivable | 3,690 | 2,100 |  |  |  |
Net proceeds from sales of loans held for investment, principal | 3,420 | 1,960 |  |  |  |
Net proceeds from sales of loans held for investment, capitalized interest | $ 274 | $ 144 |  |  |  |

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